Paramagnetic sample arrangement for ECM false echo generation

ABSTRACT

An arrangement for generating false echoes for ECM is characterized by providing identical magnetic environments for two paramagnetic samples for spin echo delay lines at microwave frequencies. A static magnetic field is formed by parallel faces of spaced-apart pole pieces having received therebetween two separate and independent waveguides. The two waveguides are arranged such that a common wall between them lies in the center plane of mirror symmetry of the magnetic field. A paramagnetic sample is disposed in each waveguide between the pole pieces such that the magnetic field distribution across the samples is substantially uniform but provides sufficient bandwidth for producing spin echoes of input microwave pulses. Circuitry is disclosed for directing an input pulse to one of the paramagnetic samples from which, at a time later, an echo is recalled and forms an input pulse to the second paramagnetic sample. A false echo is then provided by recalling the input pulse to the second paramagnetic sample at a selected time later.

States Patent [191 net;

[ Feb. 11,1975

[75] Inventor: Daniel C. Buck, Hanover, Md.

[73] Assignee: Westinghouse Electric Corporation,

' Pittsburgh, Pa.

[22] Filed: June 4, 1973 [2]] Appl. No.: 366,914

[52] U.S. Cl. 343/18 E, 324/.5 R, 333/31 A, 340/173 NC [51] Int. Cl...... H04k 3/00, GOlr 33/08, H03h 7/30 [58] Field of Search 324/.5 R, .5 AC; 333/30 M, 333/31 R, 31 A; 343/18 E; 340/173 NC [56] References Cited UNITED STATES PATENTS 2,944,246 7/1960 Dicke 340/173 Nl 3,056,080 9/1962 Crandell 324/.5 -R

3,109,138 10/1963 Varian 324/.5 R 3,147,427 9/1964 Varian 324/.5 R

3,345,620 10/1967 Anderson et a1. 340/173 NI 3,577,145 5/1971 Worden et al. 324/.5 R

3,581,190 5/1971 Brown 324/.5 R 3,732,488 5/1973 Franconi 324/.5 R

Primary ExaminerMaynard R. Wilbur Assistant Examiner-G. E. Montone Attorney, Agent, or Firm-J. B. Hinson [57] ABSTRACT An arrangement for generating false echoes for ECM is characterized by providing identical magnetic environments for two paramagnetic samples for spin echo delay lines at microwave frequencies. A static magnetic field is formed by parallel faces of spaced-apart pole pieces having received therebetween two separate and independent waveguides. The two waveguides are arranged such that a common wall between them lies in the center plane of mirror symmetry of the magnetic field. A paramagnetic sample is disposed in each waveguide between the pole pieces such that the magnetic field distribution across the samples is substantially uniform but provides sufficient bandwidth for producing spin echoes of input microwave pulses. Circuitry is disclosed for directing an input pulse to one of the paramagnetic samples from which, at a time later, an echo is recalled and forms an input pulse to the second paramagnetic sample. A false echo is then provided by recalling the input pulse to the second paramagnetic sample at a selected time later.

10 Claims, 6 Drawing Figures PATEN'I'ED 11975 3.866.225

SHEET 1 OF .3

PRECESSIONAL MOTION: y9=0 NO MICROWAVE FIELD APPLIED MAGNETIC ELECTRON SPIN MOMENT DURING APPLICATION OF MICROWAVE MAGNETIC FIELD QINCREASES WITHTIME.

EQUATOR:

9=7T RECALL V TT RECALLED SIGNAL ECHO R T 1 L 1 f1 TIME- 2 62 I /00,U$ 5;]5 3 ni ss Fig. 3 3 SPIN 2 R 9 I I (an I 64 I x r as SPIN ECHO PUILSE RADAR I 1' :1

[R EXPECTED PULSE RETURN PULSE TIME PATENTEU FEB] 1 5 Fig.4

SHEET 2 OF 3' LIMITER sPsT SPINZ 1L IOU 1s 5 5 DELAY DELAY 53b r53 5 Js 52 DELAY I 35 SPINI L 34 y E i BLOCKING -40 46 TTPULSE 43 r: l DPST 5 STAB RECALL osc.

ULSE 42 BLOCKING osc PATENTEDFEB] H975 SHEET 30F 3 DCuv H In UnitsOf MHz Wider Spectrum Than The Recall Pulse Spectrum Magnetic \Field In Units 0f Oersted 1: MHz

0. Half Amplitude 1' Of The Spectral I Distribution N Recall Pulse Spectrum Signal Pulse Spectrum Frequency PARAMAGNETIC SAMPLE ARRANGEMENT IFOR ECM FALSE ECHO GENERATION BACKGROUND OF THE INVENTION False echo generation for electronic countermeasure systems (ECM) has been accomplished in the past in several different ways. One method predominantly used involved beating down an input pulse from a microwave frequency to a video frequency range where ultrasonic delay lines are feasible. The technique of this method is cumbersome for interpulse times in the range of hundreds of microseconds and pulse train lengths of the order of milliseconds. These time parameters of pulses are such that delay lines can be effectively provided using spin echo techniques.

SUMMARY OF THE INVENTION It is an overall object of the present invention to provide identical distributions of a magnetic field for two paramagnetic samples of spin echo systems for tandem use to delay an input pulse and generate false echoes therefrom.

More specifically, it is an object of the present invention to provide an ECM false echo generator wherein an input pulse is time delayed by producing echo pulses therefrom by circulating them between two spin echo systems for the generation of a train of long delayed pulses for each input pulse.

Specifically, according to the present invention, there is provided a paramagnetic sample arrangement for ECM false echo generation comprising a magnet with pole pieces having opposed pole faces that are uniformly spaced apart to provide a static magnetic field therebetween, means forming two separate and independent waveguide chambers in the space between the pole pieces, the waveguide chambers being arranged to form a wall common between them lying in the center plane between the pole pieces, and a paramagnetic sample disposed in each waveguide chamber in a position such that each sample is subject to a substantially identical magnetic field environment by the opposed pole pieces. The magnetic field can be generated either by means of an electromagnet or a permanent magnet.

In the preferred form, a control system is provided for the spin echo system including means for delivering an input microwave pulse to one of the two spin systems, means for detecting an input pulse at a video radio frequency, means for delaying the detected radiofrequency pulse to generate a recall pulse to the spin system storing the input pulse, means for delivering an echo pulse from the first spin system to the second spin system, and a second delay means for the detected pulse at a video radio frequency for delivering a recall signal to the second spin system whereby the echo pulse therefrom is in the form of a long delayed pulse having a bandwidth and frequency characteristic corresponding to the pulse input to the first spin system. The.

invention further provides circulator means whereby the pulse from the second spin system is delivered to a transmitting antenna as well as undergoing recirculation into the first spin system as an input pulse thereto, thereby generating a train of long delayed pulses for each input pulse.

These features and advantages of the'present invention as well as others will be more apparent when the following description is read in light of the accompanying drawings, in which:

FIG. 1 is a geometrical illustration of the phenomenon of spin echoes of a nuclear or electron paramagnetic spin system;

FIG. 2 is a linear diagram illustrating the time relationship of a pulse input and an echo output from a nuclear spin system;

FIG. 3 is a time diagram of the delay provided by two spin systems for providing a spin echo delayed output pulse;

FIG. 4 is a block diagram of one form of control system for delaying a pulse using two spin echo systems;

FIG. 5 is a detailed diagram of two matched spin systems in waveguide resonators; and

FIG. 6 is a typical frequency spectra for faithful echo generation according to the present invention.

The physical phenomenon of forming the underlying basis for the present invention is called spin echo which is set forth at length in a paper entitled Spin Echoes" byE. L. Hahn, published in the Physical Review, Vol. 80, No. 4, 580-594, on Nov. 15, 1950. A detailed explanation of the phenomenon underlying spin echoes is deemed unnecessary in view of the explanation given in the Hahn publication. A greatly simplified explanation will now be given in order to enhance an understanding of the underlying principles of the present invention in which regard reference will be made to FIG. I of the drawings. It has been determined that in nuclear magnetic substances, two characteristics of the atomic nuclei are spin and magnetic moments. Each nucleus has mass, however tiny, and its spin entails angular momentum so that the nucleus acts as a small gyroscope. At the same time, since each nucleus is composed of one or more charged particles or protons, the spinning motion of these charged particles gives rise to electrical effects which impart a characteristic magnetic moment to the nucleus as a whole. In a paramagneticsubstance, the spins are formed by unpaired electrons in the host crystal lattice. These electrons also have the property of spin angular momentum, and experience precession around an applied magnetic field. For application of this invention to microwave frequencies, the electron paramagnetic spin system is used rather than the nuclear spin system because the electron mass is much smaller, giving rise to greatly increased precession frequencies. By considering the free processing electron spins in a paramagnetic substance as if they were a myriad of lightly-coupled cavities of extremely high Q factors of the order of magnitude of 10 the resonant frequency of these little cavities is then proportional to the applied magnetostatic field at the spin location in the paramagnetic substance. In FIG. 1, the electron e is illustrated with its magnetic moment processing around and making an angle 0 to the vertical axis thereof under an applied direct current field shown in FIG. 1 as I-I The axis of the precessing spin undergoes an orbital path as indicated by the line L. A microwave field H may be applied in the direc tion normal to the applied field H Under these conditions, then, the angle 0 between H and the spin magnetic moment vector is given by the expression:

0 'y H X t where t equals a given time duration of the microwave pulse and y is equal to the gyromagnetic ratio which is a function of the paramagnetic material, usually about 2.8 megahertz per oersted. When 6 is equal to 17/2 or the electron-magnetic moment has an orbit about the equator" in relation to the direction of H The frequency f of the particle e may be expressed as:

f Y H DC X 2 77 where 211' equals constant and 7 H is an average of these quantities over a period of time. It can be seen as indicated previously that the resonant frequency of these little cavities is proportional to the applied magnetostatic field at the spin location in the paramagnetic substance. By varying the applied field over a paramagnetic sample, one can set up substantially any desired bandwidth of information storage, assuming that the number of spin systems or cavities is so large that they make up an essentially continuous frequency spectrum.

Echo generation is then essentially a mechanism by which an information pulse stored in the spins can be recalled as an echo of the input pulse. Let it be assumed that a microwave input pulse is applied to a paramagnetic sample in the direction of H to produce a resonant condition at or near y I-I Now if a second input pulse whose product of H and the time gives 6 =41, called a 1r pulse, occurs time 1' later, then an echo will appear at a time 1 later after the 7r or recall pulse which is illustrated in FIG. 2. As described in the l-Iahn paper, the recall or'rr pulse has an energy per unit bandwidth which is sufficient to invert the spins in a given cavity storing the original signal pulse. By inverting the spins, there is produced the dynamic equivalent to reversing the direction of the applied field and for this reason the recall pulse is often called the pulse. Recalling of the original signal pulse stored in the spin system will ooccur with the faithfulness of frequency reproduction which depends upon the spread of the applied field values corresponding to the frequency components making up the original signal pulse. This is conditioned upon (1) that the original signal pulse is small compared to the recall pulse and (2) that the interpulse period 1' is small compared to two basic relaxation times inherent in magnetic spin systems. These basic relaxation times are conventionally referred to as T, and T, where T, is the spin-lattice relaxation time and T is the spin-spin relaxation time. The time T, is very temperature dependent and usually is of the order of a microsecond or less, while the time T is of the order of a millisecond if the spins are sufficiently far apart. As discussed previously in regard to FIG. 2, a 7r pulse is a recall pulse for an input pulse to a spin system. A 11 pulse is further defined as a pulse with an energy per unit bandwidth which is sufficient to invert the direction of the spin in a given atom. A 1r/2 pulse has an energy per unit bandwidth which is sufficient to rtate the spin magnetic moment 90 or 1r/2 radians.

It is thus apparent that spin echoes must be generated by a fundamental chain of events shown typically in FIG. 2 and in detail according to the present invention in FIG. 3. In FIG. 6 there is illustrated the frequency spectra for faithful response of echo generation. When a paramagnetic sample is chosen correctly, for every frequency f, there will be set up a spin of an electron in the sample according to the expression:

f Y nc Thus, a chosen paramagnetic sample under an applied field will have an average frequency fav 'Y DCav and a frequency spectrum characterized by fav Y ncuw) nc) usually defined at the half amplitude points of the spectral distribution as shown by the graph line 10 in FIG. 6. An input signal having an average frequency shown by the graph line 11 will have a Af less than the frequency spectra of both the applied field Af graph line 10, and a recall signal Af i.e., pulse, given by the graph line 12. Thus, it will be observed that both the applied field and the recall pulse have a frequency spectrum exceeding that of the signal to be recalled. In order to provide an efficient spin system, matched spectra of response (f,,,,) are required, i.e., Af =Af Af For example, in order to store and recall a signal with a 100 megahertz bandwidth, there would be needed an applied field spread of 100/28 megahertz per oersted which is equal to 35.8 oersteds about the average value. The fractional sample volume within the field at any given value is given by the amplitude of the applied field spectral density distribution.

Another form of spin echo generation is called the simulated spin echo and in this system after an input signal there is introduced a 1r/2 pulse to rotate the spins by 90 or 1r/2 radians. At a time 1', later, another 'n'/2 pulse is introduced which, in turn, recalls an echo of the original input signal. This concept, which is described in detail by the aforementioned Hahn publication, has the feature that the interpulse period 'rr need be longer than T, but not T Such a simulated echo is useful in the ECM false echo generation systems. By

7 employing two spin echo systems, a train of long delayed pulses may be produced for each input pulse. In order to achieve such a concept with usable results, it is necessary to create an identical magnetic environment for two paramagnetic materials used in each of the two spin echo systems.

As illustrated in FIG. 5, the present invention provides an apparatus in the form of opposed pole pieces I 20 and 21 with the pole faces thereof arranged in a uniformly spaced-apart relation forming a magnetic field between the pieces. It will be observed that while the magnetic field is not completely uniform in the space between the pieces and particularly at the outer edges, there does exist a plane P parallel to the faces of the pole pieces such that the magnetic field between the pole pieces with respect to the plane has mirror symmetry. Arranged symmetrically along this plane is a central wall 22 forming the floor of a waveguide cavity 23 and the ceiling of a waveguide cavity 24. An end wall 22A encloses one end of each of the waveguide cavi ties. A top wall 25 forms the ceiling for the cavity 23 and a wall 26 forms the floor for the cavity 24. Side walls 27, only one shown in FIG. 5, complete the waveguides. A paramagnetic sample 28 is located in the cavity 23 and a paramagnetic sample 29 is located in the cavity 24. The location of these paramagnetic samples is such that they are equally positioned within the waveguide cavities so that each experiences the same magnetic field between the pole pieces. It is apparent from FIG. 5 that the waveguide chambers are superimposed such that chamber 23 overlies chamber 24 with the wall 22 therebetween. The actual paramagnetic material which may be used, for example, may take the form of fused silica after it has been irradiated with gamma rays. Thus, it will be observed in regard to FIG. 5 that the present invention has as one of its principal features attributing to achieving usable results is the creation of an identical magnetic environment for the two paramagnetic materials, thereby providing two spin echo systems. These spin echo systems are shown in FIG. 4 as a spin system 30 and a spin system 31.

FIG. 4 illustrates one form of control circuit for the spin systems including an antenna 32 for receiving an input pulse such as that delivered by a radar set. The pulse from the antenna is delivered through a circulator 33 to a second circulator 34 where the radar input pulse is transferred along waveguide 35 to spin system 36. As this occurs, a coupler 36 forms a branch line for the input pulse to a detector 37 converting the input microwave pulse to a video frequency spectrum and delivering it to a 5 microsecond delay line 38 which, in turn, produces a pulse in line 39. The delayed pulse in line 39 is received by a blocking oscillator 40 which, in turn, applies the pulse to the grid ofa microwave amplifier tube 41 such as a traveling wave tube having a stabilized microwave oscillator 42 connected thereto.

The traveling wave tube 43 produces a very sharp pulse in waveguide 44. This sharp pulse passes through a double-pole, single-throw switch 45 and takes the form of a 11' pulse delivered by a coupler 46 to the waveguide 35 forming a 11' recall pulse for the spin system 30. At the same time, the very sharp pulse in waveguide 44 is connected by a coupler 47 to waveguide 48 and due to the losses incurred during the coupling action, the pulse takes the form of a 17/2 pulse. This pulse is delivered through a microwave amplifier 49 and thence to a circulator 50 for delivery to spin system 31. Thus, the 11' pulse in the spin system 30 and the 11/2 pulse in the spin system 31 are 5 microsecond delayed microwave pulses.

The 1r pulse in the spin system 30 recalls the radar input pulse from this system 5 microseconds later or microseconds after it passes into the spin system 30. The echo pulse is transferred through circulator 34 through a single-pole, single-throw switch 51 which is rendered conductive in response to a pulse in line 52 delivered from a 5 microsecond delay line 53 which re ceives as its input signal the delayed pulse in line 39. Thus, it will be observed that the echo pulse from the spin system occurs 10 microseconds after its entry into the system. This echo pulse travels along waveguide 48 to the circulator 50 where it is transferred to the spin system 31. At this point, in system 31 we have a 1r/2 pulse and 5 microseconds later an input signal pulse.

In order to provide a recall pulse in the form ofa 11/2 pulse for the spin system 31, the 5 microsecond delayed pulse in line 39 is connected to :1 I00 microsecond dealy line 53a which produces a 105 microsecond total time delay pulse in line 53b which is applied to the blocking oscillator as well as a blocking oscillator 54. The pulse in line 53b passes through the blocking oscillator 40 and thence through the traveling wave tube 41 producing in waveguide 44 which is transferred through the coupler 47 as a 17/2 pulse to the waveguide 48 where it travels into the spin system 31 by the action of the circulator 50. An echo pulse is produced 5 microseconds later which travels from the spin system 31 through circulator 50 and through a single-pole, singlethrow switch 55 which has been rendered conductive by the output signal from blocking oscillator 54. The echo pulse then travels through a waveguide 56 into a signal limiter 58 and thence through an amplifier 59 from where it is discharged to the circulator 33. The circulator delivers a major portion of the delayed echo pulse to the antenna 32 while due to leakage a portion of this pulse is again transmitted to the circulator 34. In this manner, it is possible to recirculate the delayed echo pulse again between the two spin systems to allow the generation of a train of long delayed microwave pulses for each radar pulse input.

With reference now to FIG. 3, there is illustrated a time relation of the various pulses in the spin systems 30 and 31 which may be typically carried aboard an aircraft having the antenna 32 forming part thereof. Based on the foregoing description in regard to FIG. 4, it can be seen that at a time T a microwave radar pulse may be produced from the ground or other source which is received by the antenna 32 as an input pulse to the spin system 30. Through the use of the 5 microsecond delay 38, a recall pulse 60 is produced whereby the echo from the spin system 30 occurs 5 microseconds later at 61. At the same time the recall pulse 60 is provided, a 1r/2 pulse 62 is transferred to the spin system 31 which additionally receives 5 microseconds later the echo pulse 61 from the spin system 30.. This echo pulse appears as an input pulse 63 which is then recalled I00 microseconds later by the 11/2 pulse 64 to produce a false echo 65 S-microseconds after the pulse 64. A portion of the echo pulse is then transmitted by the antenna 32 to the radar set forming a spin echo input pulse 66 which is delayed some preselected time X later than the expected return of the radar pulse had the original radar pulse not undergone the time delays.

Thus, by providing the two spin systems which undergo the same applied field spectra as shown in FIG. 6, there is preserved the precise pulse spectrum each time an echo is generated. In order to accomplish this, the two spin systems are located in reduced height waveguide resonators as shown by the arrangement of FIG. 5 whereby each spin system lies on opposite sides of the center plane of mirror symmetry of the applied field. This could also be done with a microstrip, stripline, or paired coaxial configurations so long as the two circuits and the spin system are isolated.

Although the invention has been shown in connection with a certain specific embodiment, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts and allocation of recall times may be made to suit requirements without departing from the spirit and scope of the invention.

What is claimed is: 1. An apparatus for providing spin echo delay lines for microwave signal pulses, said apparatus being characterized by two paramagnetic samples at opposite sides of a plane of symmetry passing through a magnetic field, said apparatus comprising:

pole pieces having uniformly-spaced pole faces forming a magnetic field having a plane of symmetry passing along the space between the pole faces,

means defining two separate and superimposed waveguide chambers such that one waveguide chamber overlies the other in the space between said pole pieces, said waveguide chambers being arranged to define a wall extending between them along said plane of symmetry of the magnetic field between said pole pieces, and

paramagnetic means disposed in each waveguide chamber in a position such that with respect to said plane of symmetry the magnetic field between said pole pieces is symmetrically applied to each paramagnetic means.

2. The apparatus according to claim 1 wherein said waveguide chambers are further defined to include a 6. The apparatus according to claim 1 further includ- 7 V ing conduit means for delivering an input pulse to said paramagnetic means disposed in one of said waveguide chambers, means for detecting the input pulse at a video radio frequency, first delay means for the detected video radio frequency pulse to generate a pulse to recall a time delayed echo of the input pulse, means for delivering the echo pulse to said paramagnetic means disposed in the other of said waveguide chambers, and a second time delay means for the detected pulse at a video radio frequency for delivering a recall pulse to said other of said waveguide chambers whereby a twice time delayed echo pulse therefrom has a bandwidth and frequency characteristic corresponding to said pulse input.

'7. The apparatus according to claim 6 further comprising antenna means for receiving microwave frequency signal pulses to form said input pulse, circulator means for directing said input pulse to said one of the waveguide chambers, said circulator means receiving said twice time delayed echo pulse for discharging a portion thereof to said antenna means and discharging a portion thereof to said waveguide means thereby forming an input pulse.

8. The apparatus according to claim 7 further comprising amplifier means for said pulse from said first time delay means, a traveling wave tube for producing microwave frequency pulses from the output from said amplifier means, and coupler means for delivering said microfrequency pulse to both of said waveguie chambers.

9. The apparatus according to claim 8 further comprising a blocking oscillator for receiving said first time delayed pulse for delivery to said amplifier means, said blocking oscillator further receiving the pulse from said second time delay means, and switch means in the signal path from said traveling. wave tube for limiting delivery of said recall pulse to said other of said waveguide chambers.

10. The apparatus according to claim 1 further comprising, means to recall spin echoes of microwave input pulses from the paramagentic means in one of said waveguide chambers for delivery as microwave spin echo input pulses to the paramagnetic means in the other of the waveguide chambers. 

1. An apparatus for providing spin echo delay lines for microwave signal pulses, said apparatus being characterized by two paramagnetic samples at opposite sides of a plane of symmetry passing through a magnetic field, said apparatus comprising: pole pieces having uniformly-spaced pole faces forming a magnetic field having a plane of symmetry passing along the space between the pole faces, means defining two separate and superimposed waveguide chambers such that one waveguide chamber overlies the other in the space between said pole pieces, said waveguide chambers being arranged to define a wall extending between them along said plane of symmetry of the magnetic field between said pole pieces, and paramagnetic means disposed in each waveguide chamber in a position such that with respect to said plane of symmetry the magnetic field between said pole pieces is symmetrically applied to each paramagnetic means.
 2. The apparatus according to claim 1 wherein said waveguide chambers are further defined to include a wall defining a floor for one chamber and a roof for the other chamber, said wall lying within said plane of symmetry.
 3. The apparatus according to claim 2 wherein said waveguide chambers further include walls for enclosing said first-mentioned wall and defining therewith said waveguide chambers.
 4. The apparatus according to claim 1 wherein said paramagnetic means includes fused silica.
 5. The apparatus according to claim 4 wherein said paramagnetic means is further defined as fused silica irradiated with gamma rays.
 6. The apparatus according to claim 1 further inclUding conduit means for delivering an input pulse to said paramagnetic means disposed in one of said waveguide chambers, means for detecting the input pulse at a video radio frequency, first delay means for the detected video radio frequency pulse to generate a pulse to recall a time delayed echo of the input pulse, means for delivering the echo pulse to said paramagnetic means disposed in the other of said waveguide chambers, and a second time delay means for the detected pulse at a video radio frequency for delivering a recall pulse to said other of said waveguide chambers whereby a twice time delayed echo pulse therefrom has a bandwidth and frequency characteristic corresponding to said pulse input.
 7. The apparatus according to claim 6 further comprising antenna means for receiving microwave frequency signal pulses to form said input pulse, circulator means for directing said input pulse to said one of the waveguide chambers, said circulator means receiving said twice time delayed echo pulse for discharging a portion thereof to said antenna means and discharging a portion thereof to said waveguide means thereby forming an input pulse.
 8. The apparatus according to claim 7 further comprising amplifier means for said pulse from said first time delay means, a traveling wave tube for producing microwave frequency pulses from the output from said amplifier means, and coupler means for delivering said microfrequency pulse to both of said waveguie chambers.
 9. The apparatus according to claim 8 further comprising a blocking oscillator for receiving said first time delayed pulse for delivery to said amplifier means, said blocking oscillator further receiving the pulse from said second time delay means, and switch means in the signal path from said traveling wave tube for limiting delivery of said recall pulse to said other of said waveguide chambers.
 10. The apparatus according to claim 1 further comprising, means to recall spin echoes of microwave input pulses from the paramagentic means in one of said waveguide chambers for delivery as microwave spin echo input pulses to the paramagnetic means in the other of the waveguide chambers. 